Geobiology (2013)
DOI: 10.1111/gbi.12045
Carbon isotopes and lipid biomarkers from organic-rich
facies of the Shuram Formation, Sultanate of Oman
C. LEE,1,2 D. A. FIKE,3 G. D. LOVE,1 A. L. SESSIONS,2 J. P. GROTZINGER,2
R. E. SUMMONS4 AND W. W. FISCHER2
1
Department of Earth Sciences, University of California, Riverside, CA, USA
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
3
Earth and Planetary Sciences, Washington University St Louis, St. Louis, MO, USA
4
Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
2
ABSTRACT
The largest recorded carbon isotopic excursion in Earth history is observed globally in carbonate rocks of
middle Ediacaran age. Known from the Sultanate of Oman as the ‘Shuram excursion’, this event records a
dramatic, systematic shift in d13Ccarbonate values to ca. 12&. Attempts to explain the nature, magnitude
and origin of this excursion include (i) a primary signal resulting from the protracted oxidation of a large
dissolved organic carbon reservoir in seawater, release of methane from sediment-hosted clathrates, or
water column stratification; and (ii) a secondary signal from diagenetic processes. The compositions and
isotope ratios of organic carbon phases during the excursion are critical to evaluating these ideas; however,
previous work has focused on localities that are low in organic carbon, hindering straightforward interpretation of the observed time-series trends. We report carbon isotope data from bulk organic carbon, extracted
bitumen and kerogen, in addition to lipid biomarker data, from a subsurface well drilled on the eastern
flank of the South Oman Salt Basin, Sultanate of Oman. This section captures Nafun Group strata through
the Ediacaran–Cambrian boundary in the Ara Group and includes an organic-rich, deeper-water facies of
the Shuram Formation. Despite the high organic matter contents, the carbon isotopic compositions of carbonates – which record a negative d13C isotope excursion similar in shape and magnitude to sections elsewhere in Oman – do not covary with those of organic phases (bulk TOC, bitumen and kerogen). Paired
inorganic and organic d13C data only display coupled behaviour during the latter part of the excursion’s
recovery. Furthermore, lipid biomarker data reveal that organic matter composition and source inputs
varied stratigraphically, reflecting biological community shifts in non-migrated, syngenetic organic matter
deposited during this interval.
Received 28 January 2013; accepted 24 May 2013
Corresponding authors: C. Lee. Tel.: +14242488232; fax: +19518274324; e-mail: [email protected]
W. W. Fischer. Tel.: +16263956790; fax: +16265680935; e-mail: [email protected]
INTRODUCTION
The rock record of the late Neoproterozoic Era (ca. 800–
541 Ma) captures a dynamic time in Earth history, marked
by several global low-latitude glaciations (Hoffman et al.,
1998), increasing and widespread oxygenation of Earth’s
atmosphere and oceans (Knoll et al., 1986; Fike et al.,
2006; Scott et al., 2008), and the evolution and diversification of Metazoa (Knoll & Carroll, 1999). Subsequent to
the Cryogenian global glaciations yet prior to the evolution
of bilateria near the Ediacaran–Cambrian boundary, middle
© 2013 John Wiley & Sons Ltd
Ediacaran-age successions host the largest negative carbon
isotopic excursion observed in the geologic record, with
values of d13Ccarbonate (d13Ccarb) as low as 12& (summarised in Grotzinger et al., 2011). This excursion was first
observed in the Wonoka Formation of South Australia
(Jansyn, 1990; Pell et al., 1993; Urlwin et al., 1993).
Burns & Matter (1993) were the first to detail the unusual
excursion in Oman and link these carbon cycle dynamics
to animal evolution. In addition to Oman (e.g. Le
Guerroue et al., 2006a,b,c) and Australia (e.g. Calver,
2000), negative excursions displaying similar characteristics
1
2
C. LEE et al.
have been observed in many Ediacaran sedimentary successions, including the Doushantuo Formation of South
China (e.g. McFadden et al., 2008) and the Johnnie
Formation of the western United States (e.g. Kaufman
et al., 2007; Bergmann et al., 2011).
If primary, the striking pattern observed in the Shuram
excursion stretches our ability to interpret the historical
behaviour of the carbon cycle using commonly applied
assumptions in an isotope mass balance framework. Fundamentally, explanations must invoke carbon fluxes into the
fluid Earth with isotope ratios lower than commonly
assumed for outgassing and weathering. Alternatively, it
has been hypothesised that the Shuram excursion does not
record the primary isotopic composition of marine dissolved inorganic carbon (DIC), but was caused rather by
secondary, diagenetic processes – perhaps global in scope
(Bristow & Kennedy, 2008; Knauth & Kennedy, 2009;
Derry, 2010a; Grotzinger et al., 2011; Schrag et al.,
2013). Several observations fuel diagenetic hypotheses –
including the widely observed positive correlation between
the carbon and oxygen isotopic composition of carbonates
– but another critical part of the Shuram puzzle is that the
isotopic composition of coeval organic matter does not
record the same time-series pattern, but is instead largely
invariant (e.g. Fike et al., 2006; McFadden et al., 2008).
This feature might be most simply explained if the excursion were diagenetic in origin (e.g. Derry, 2010b), but
two different hypotheses have been developed to explain
the pattern as a consequence of primary sedimentary and
carbon cycle processes.
Focusing on the pattern of decoupled d13Ccarb and
13
d Corg values in Neoproterozoic successions, Rothman
et al. (2003) formulated a generic carbon cycle scenario in
which both the negative excursions in d13Ccarb values and
the invariant organic matter d13C values (d13Corg) were
caused by the slow oxidation of a large dissolved organic
carbon (DOC) seawater pool (Rothman et al., 2003). A
large DOC reservoir, much larger than the mass of inorganic carbon in DIC, allows d13Corg to be buffered against
isotopic variation, while its progressive and eventual remineralisation would have produced 13C-depleted isotope
ratios observed in carbonate (Rothman et al., 2003). Fike
et al. (2006) subsequently invoked this hypothesis to
explain the Shuram excursion, and this idea has seen broad
application to explain decoupled carbonate–organic data
sets from other sections (e.g. McFadden et al., 2008) and
Neoproterozoic intervals (e.g. Swanson-Hysell et al.,
2010), and has been integrated into new modelling
approaches (Bjerrum & Canfield, 2011). A reasonable concern with the decoupling argument comes from the quality
of the organic carbon isotope data, much of which comes
from organic-lean sections (Calver, 2000; Fike et al.,
2006; Swanson-Hysell et al., 2010) – with TOC wt% values of <0.05, perhaps approaching analytical blanks.
Recently, it was proposed that throughout Neoproterozoic
time, d13Corg values should record an expected 13Cdepleted linear translation of DIC trends and that the reason organic-lean samples do not recapitulate the expected
trends is because the d13Corg of these samples is instead
controlled by admixtures of fossil detrital or migrated
organic carbon (Johnston et al., 2012). In order to test
these different hypotheses, it is important to examine the
isotopic systematics of strata that capture the Shuram
excursion that are rich in organic carbon and derived from
unweathered materials.
Much of the Shuram Formation studied thus far is
organic lean in both outcrop and subsurface strata, but in
certain areas of the basin, petroliferous and fossiliferous
strata are preserved. The eastern flank of the South Oman
Salt Basin (Fig. 1A) contains extremely thermally wellpreserved molecular fossils (e.g. Love et al., 2008, 2009;
Grosjean et al., 2009) as well as a suite of sphaeromorphic
acritarchs and filamentous microfossil assemblages in the
Nafun Group (Butterfield & Grotzinger, 2012). Here, we
report isotopic and lipid biomarker data from an eastern
flank locality that captures a deeper-water (Fig. 1B),
organic-rich facies of the Shuram and overlying Buah
formations, in order to better understand carbon cycle
processes and biological community dynamics through the
Shuram excursion.
Geological setting
The Huqf Supergroup of the Sultanate of Oman provides
one of the best-preserved, most continuous late Neoproterozoic to early Cambrian (ca. 713–540 Ma; Allen,
2007; Bowring et al., 2007) successions globally. Outcrop
exposures of Huqf strata are limited to the Oman Mountains in the north, Mirbat area in the south, and the
Huqf area, but a substantial amount of knowledge and
materials exist from numerous petroleum exploration and
production wells throughout the subsurface, particularly
the South Oman Salt Basin (Schr€
oder et al., 2004)
(Fig. 1).
The Huqf Supergroup is composed of three groups that
show broadly similar stratigraphic patterns between the
sub-basins. The lowest group – the Abu Mahara – is composed of clastics, including Cryogenian-age, glaciallyderived sediments (Allen, 2007). The overlying Nafun
Group begins with the Marinoan (ca. 630 Ma) Hadash cap
carbonate and then records two overall clastic-to-carbonate
shallowing upward trends defined by the Masirah Bay clastics and overlying Khufai Formation carbonate ramp, and
the Shuram Formation mixed clastics and carbonates with
the overlying Buah Formation carbonate ramp (McCarron,
2000; Grotzinger et al., 2002; Cozzi et al., 2004; Le
Guerroue et al., 2006a). The Shuram excursion nearly
covers one of these first-order cycles, beginning in the
© 2013 John Wiley & Sons Ltd
Organic carbon through the Shuram Formation
A
Iran
26°
B
3
26°
Persian Gulf
Gulf of Oman
24°
m
an
Mt
s.
d
hu sin
Fa t Ba
l
lt
Sa
Sa
ba in
ha as
G B
lt
Sa
ba in
ha as
G B
22°
22°
Hu
qf
Saudi Arabia
24°
Muscat
O
United Arab
Emirates
So
u
Sa th O
lt
Ba ma
sin n
20°
20°
Arabian Sea
subsurface wells
outcrop sections
18°
18°
TM6
TM6
shallow to deep facies
100 km
54°
56°
58°
100 km
54°
56°
58°
Fig. 1 Left panel: geologic map of the sub-basins containing Huqf Supergroup strata throughout the Sultanate of Oman. Outcrop localities are shown in dark
blue; subsurface salt basins are shown in orange. The location of the subsurface well TM-6 on the eastern flank of the South Oman Salt Basin is marked.
Right panel: bathymetric facies map of the Shuram Formation in Oman (data from Petroleum Development Oman and Le Guerroue et al., 2006a).
uppermost Khufai Formation (Osburn et al., 2013) with
the recovery finishing in the lower Buah Formation (Burns
& Matter, 1993; Le Guerroue et al., 2006a). Finally, the
Ara Group records an evaporite basin with several successive carbonate platform stringers (Gorin et al., 1982;
Schr€
oder et al., 2004). Despite similar overall stratigraphic
patterns between basins, facies differences between regions
record striking gradients in paleoenvironment; Nafun
Group strata exemplify this.
The Shuram Formation exhibits significant facies
changes across Oman. In outcrop, it is well known that
shallow shelf platform facies in the Huqf outcrop region
pass downdip into sub-wave base deposits (McCarron,
2000; Le Guerroue et al., 2006a,b). The depositionally
updip deposits include wave-rippled to hummocky crossstratified siltstones and fine sandstones, interstratified with
trough cross-stratified ooid grainstones and intraclast
© 2013 John Wiley & Sons Ltd
conglomerates. Characteristically these upper shoreface
deposits form shallowing-upward cycles in which the siliciclastic-dominated facies pass upward into the carbonatedominated facies (Le Guerroue et al., 2006b).
In contrast, downdip facies are expressed as interbedded siliciclastic mudstones and siltstones, with zones of
carbonate cementation and concretion development. Convolute bedding and slump structures are present, along
with current ripples. Uncommonly, small-scale hummocky
cross-stratification is observed. References to ‘organic-rich’
facies in outcrop have been noted (e.g. Le Guerroue
et al., 2006a) though these have not been confirmed
through measurement of organic carbon concentrations.
Due to the lack of surface oxidative weathering, the subsurface record of Shuram Formation facies variability provides the best indicator of how primary concentrations of
organic content vary spatially and temporally throughout
4
C. LEE et al.
the formation. Well data collected by Petroleum Development Oman over the past 40 years were examined by Le
Guerroue et al. (2006a) who reproduced their facies map
for the Shuram Formation (Fig. 1B). Figure 1B shows
these lateral facies changes in the Shuram, illustrating the
presence of deeper-water facies trending NE–SW, shallowing to the east along the Huqf area outcrop belt, a longlived topographic high. The western limit of the deeperwater facies belt is uncertain due to uplift and erosion
along the Western Deformation Front (Loosveld et al.,
1996; Grotzinger et al., 2002). Paleogeographic trends
suggest it would have continued to deepen in a westerly
direction (see Allen, 2007, Fig. 10).
Only a small fraction of the Shuram Formation has
been cored. Fortunately, it is standard industry practice to
analyse cuttings for information regarding lithology,
including primary sedimentology, diagenesis and the
detection of organic-rich facies. Wireline logs additionally
provide a wealth of petrophysical data that can provide
important constraints on organic content. The presence
of organic matter can be confidently detected with a
combination of increasing gamma ray values, sonic transit
time, neutron porosity and resistivity, and with reduction
in the formation bulk density (Meyer & Nederlof, 1984;
Mann et al., 1986; Herron & Le Tendre, 1990). Based
on analysis of wireline logs and organic geochemical analyses of cuttings, the Shuram Formation was recognised to
contain organic-rich source rocks in deeper-water paleogeographic settings (Le Guerroue et al., 2006a; Grosjean
et al., 2009). We examined materials from the TM-6 well
located within this organic-rich, deeper-water facies belt
(Fig. 1B).
METHODS
Sample preparation and bitumen extraction
Samples from TM-6, collected as cm-sized drill cuttings,
were initially soaked and rinsed 39 in distilled deionised
(DI) water, then rinsed with 39 methanol, 39 dichloromethane and 39 n-hexane to remove residual surface contamination. The cuttings were milled to fine powder using
a SPEX 8510 Shatterbox with an alumina ceramic container.
Rock powders (typically 1.5–3 g) were extracted in
20 mL of dichloromethane:methanol (9:1 v/v) (CEM
MarsXpress) at 100 °C for 15 min with constant stirring.
One blank consisting of pre-baked silica sand was extracted
with each batch of samples. After cooling, sample extracts
were filtered and solvent was evaporated under a stream of
pure N2 into pre-tared vials. To ensure complete extraction, the remaining sediment was re-extracted – up to
eight times – until the extract contained no more measurable (by weight) bitumen.
Kerogen isolation and Total Organic Carbon (TOC)
To isolate kerogen, solvent-extracted rock powder was
acidified at room temperature using concentrated hydrochloric acid (1 N) for 48 h to remove carbonate minerals.
The powders were rinsed repeatedly with DI water until a
pH range of 4–5 was achieved. To prepare samples for
TOC measurements, whole-rock power was acidified as
described above. Samples were dried overnight and loaded
into capsules for analysis.
Isotope ratio analyses
Carbonates
Carbonate carbon (d13Ccarb) and oxygen isotopes
(d18Ocarb) were measured on CO2 after reaction with
anhydrous phosphoric acid by gas-source mass spectrometry according to standard methods previously described in
Ostermann & Curry (2000). Data are reported in the
conventional d13C notation as parts per thousand (permil)
deviations from the VPDB standard. The sulphur isotopic
composition of carbonate-associated sulphate (CAS) was
determined by dissolution of carbonate and precipitation
of barite, followed by combustion to SO2 and analysis by
gas-source mass spectrometry; the details of the method
are outlined in Fike & Grotzinger (2008). Briefly, CAS
was obtained by dissolving powered sample in 6 N HCl
for either 2 h at 60 °C under nitrogen gas or 12–24 h at
room temperature. Samples were filtered to eliminate
insoluble residues, and excess barium chloride was added
to the filtrate to precipitate barium sulphate for subsequent analysis. Sulphur isotope ratio data are reported in
permil relative to the VCDT standard. Samples were calibrated using the international standards NBS-127
(20.3&) and S3 ( 31.5&), as well as four internal standards: silver sulphide (ERE-Ag2S: 4.3&), chalcopyrite
(EMR-CP: 0.9&) and two barium sulphate standards
(BB4-18: 39.5&; PQB: 38.0&) (Fike & Grotzinger,
2008).
Organics
TOC and kerogen fractions were loaded into tin capsules
(Costech) as powder. Bitumen dissolved in dichloromethane was pipetted into tin capsules and the solvent was
evaporated to dryness prior to analysis. Samples for bitumen and kerogen were flash-combusted in a 1000 °C furnace using a Costech Analytical Technologies Elemental
Analyser (EA). The resulting CO2 gas was analysed by continuous flow using a Delta-S Isotope Ratio Mass Spectrometer (IRMS). Samples for TOC were flash-combusted at
1060 °C in a Carlo Erba NA1500 Elemental Analyser fitted with an AS200 autosampler. The resulting CO2 gas
was analysed by a Delta-plus XP IRMS. Organic carbon
isotope ratios are reported in the conventional delta
© 2013 John Wiley & Sons Ltd
Organic carbon through the Shuram Formation
notation as permil variations relative to the VPDB standard. Calibration of d13Corg values was accomplished by
comparison with system blanks (empty tin capsules) and
two working standards: acetanilide (C6H5NH(COCH3))
and urea (CH4N2O). Blanks and standards were run after
every ten sample measurements. Sample d13C values were
repeatable to a standard error of 0.26& (TOC), 0.32&
(bitumen) and 0.54& (kerogen). Standards were repeatable to a standard error of 0.08& (acetanilide standard;
n = 20) and 0.06& (urea standard; n = 23).
Gas Chromatography–Mass Spectrometry (GC-MS) and
Multiple Reaction Monitoring (MRM)
Saturated hydrocarbons were analysed by gas chromatography–mass spectrometry (GC-MS) in full scan mode on a
Micromass Autospec Ultima equipped with a HP6890 gas
chromatograph (Hewlett Packard). Sterane and hopane
biomarkers were analysed on the Autospec instrument by
multiple reaction monitoring (MRM). Details of the run
conditions and uncertainties in the biomarker ratios are the
same as in Love et al., 2009. Uncertainties in polycyclic
biomarker ratios determined from multiple analyses of a
saturated hydrocarbon fraction from AGSO standard oil
are 8%.
Lipid biomarkers
Traces of elemental sulphur were removed from bitumen
extracts using activated copper turnings. The extract was
separated into three fractions by columns filled with drypacked silica gel. The saturates fraction was eluted with nhexane, the aromatics with n-hexane:dichloromethane (4:1
v/v) and the polars with dichloromethane:methanol (3:1
v/v). A deuterated C29 sterane standard (d4-aaa-24-ethylcholestane (20R), Chiron Laboratories AS) was added to
the saturate fraction prior to biomarker quantification
through GC-MS. Typically, 50 ng of internal standard was
added to a 1-mg aliquot of saturates.
25
50
RESULTS AND DISCUSSION
Figure 2 contains a compilation of geochemical and isotope ratio profiles spanning ca. 1000 m of the Huqf
Supergroup strata, ranging from the lower Masirah Bay siliciclastics into Ara Group carbonates. d13Ccarb values
display two well-defined negative excursions (Fig. 2B). The
first begins near the contact of the Khufai and Shuram
formations, with d13Ccarb values dropping from +5&
δ18Ocarb‰VPDB
Carbonate (wt%)
0
75 100
–15 –10 –5
0
5
δ34SCAS‰VCDT
TOC (wt%)
5
0
5
10
15
0
20
15
30
45
60
ARA
A5-A6
A4
A0-A3
SE + recovery
Shuram
SE
100 m
NAFUN
Buah
Khufai
Masirah
Bay
A
Hadash
B
–15 –10 –5
Limestone
Siltstone & shale
Dolostone
Sandstone
0
δ13Ccarb‰VPDB
C
5
D
–45 –40 –35 –30 –25
δ13Corg‰VPDB
E
F
20
25
30
35
G
40
Δ13Ccarb-org‰
Fig. 2 Isotopic and geochemical profiles from TM-6 plotted alongside stratigraphy. Lithologic log and group and formation names are plotted along the left.
(A) Carbonate content measured in weight per cent (wt%). Note that well cuttings mix the interbedded siliciclastic and carbonate lithologies typical of the
Shuram Formation. (B) Carbon isotopic composition of carbonate, both dolomite and limestone in & VPDB. (C) Oxygen isotopic composition of carbonate in
&VPDB. (D) Carbon isotopic composition of bulk organic carbon in &VPDB. (E) Total organic carbon content measured in weight per cent (wt%). (F) Δcarb13
13
org, calculated as the difference between d Ccarb and d Corg values in &VPDB. (G) Sulphur isotopic composition of carbonate associated sulphate (CAS) in
&VCDT. Pale blue shaded panel indicates the Shuram excursion (SE) and recovery; the line in panel B represents the +5& to 12& excursion.
© 2013 John Wiley & Sons Ltd
6
C. LEE et al.
to 12& over ca. 70 m of stratigraphy with the recovery
taking place over the overlying ca. 400 m into the Buah
Formation. Even in this deeper depositional environment
with far higher sedimentary organic carbon loadings, this
d13Ccarb excursion is very similar in shape and magnitude
to the observations presented in Burns & Matter (1993),
and elsewhere in Oman (Fike et al., 2006; Le Guerroue
et al., 2006a,c). A second excursion occurs stepwise in the
A4 unit of the Ara Group, is smaller in magnitude and has
been interpreted to mark the Ediacaran–Cambrian boundary at 541 Ma (Amthor et al., 2003; Schr€
oder et al.,
2004; Bowring et al., 2007). Both carbonate carbon isotope excursions do not correlate with wt% carbonate
(Figs 2A,B and 5B).
d18Ocarb reveals a wide range of values (from 0 to 9&)
characteristic of differential alteration during carbonate diagenesis (Fig. 2C). d18Ocarb values generally decline with
stratigraphic depth, suggesting carbonate recrystallisation
during burial, although this interpretation is non-unique
without independent constraints on recrystallisation temperatures or fluid compositions. It is notable, however, that
we do not observe the strong covariation of 13C and 18O
isotope ratios in carbonates observed elsewhere in Oman
through the Shuram excursion (e.g. Burns & Matter,
1993; Fike et al., 2006; Fike, 2007), similar to observations from Le Guerroue & Cozzi (2010) and potentially
other correlative sections (e.g. Bergmann et al., 2011;
Loyd et al., 2013). This result suggests the operation of a
different combination of diagenetic processes in the carbonates deposited in this deeper-water paleoenvironment
and is not readily explained by existing diagenetic hypotheses for the Shuram carbon isotope excursion (e.g. Bristow
& Kennedy, 2008; Knauth & Kennedy, 2009; Derry,
2010a).
Bulk organic carbon isotope ratios (d13Corg) (Fig. 2D)
show a wide range of values from 28 to 39&, and several systematic trends within the stratigraphy. A broad negative excursion covers much of the Nafun Group strata,
beginning in the lower Khufai Formation and ending at
the top of the Buah Formation. Superimposed on this
trend is a smaller positive excursion in the lower Shuram
Formation. Neither d13Corg or d13Ccarb data show significant relationships with organic carbon concentrations
(TOC wt%) (d13Corg m = 1.6, R2 = 0.05 and d13Ccarb
Fig. 5D), which are generally high throughout the stratigraphy (4.4 3.8 wt%; Fig. 2E).
The trends observed in d13Corg do not directly mirror
those of d13Ccarb (Fig. 5C). The fractionation between carbonate and organic carbon (Dcarb-org; Fig. 2F) is not constant, but rather exhibits a clear systematic pattern that
mimics the Shuram excursion. Typical average marine values during the last 800 Ma for D13carb-org are in the range
of 28–32& (Hayes et al., 1999; Ader et al., 2009).
Dcarb-org values from TM-6 show a range between 24 and
39& and an average value of 32 3.5&. During the
nadir of the Shuram excursion, Dcarb-org values are
25 0.6&, whereas from the middle Shuram to upper
Buah and A4 units in the Ara, average Dcarb-org values are
33 1.9&. Finally, we do not observe the relationship
between Dcarb-org and TOC values hypothesised by
Johnston et al. (2012).
The sulphur isotopic composition of carbonateassociated sulphate (d34SCAS) shows an overall general positive trend from a nadir in the middle Shuram Formation
to high d34S values (to 40&) in the Ara Group (Fig. 2G),
observed throughout Oman (Fike & Grotzinger, 2008).
The lower Nafun Group strata reveal high d34SCAS values
in the lower Khufai Formation, which subsequently drop
by approximately 10&, reversing course briefly with a
small positive trend at the onset of the Shuram excursion.
Superimposed on these trends are 34S-depleted values that
stand out from the trend and might reflect alteration by
secondary processes (Fike & Grotzinger, 2008), possibly
including contamination by coexisting pyrite during CAS
extraction (e.g. Marenco et al., 2008). Overall, the d34SCAS
trends associated with the Shuram excursion in TM-6 are
similar to those observed by Fike et al. (2006) from shallower paleoenvironments north of the Huqf area.
Organic geochemical data through the Shuram excursion
in TM-6 show a range of features typical of the Huqf
Supergroup throughout Oman. Characteristic features of
organic matter from source rocks throughout the Huqf
Supergroup include conspicuous 13C-depletions (d13C ca.
36&), high sulphur content, low pristane/phytane, high
relative abundance of C29 steranes, lack of rearranged diasteranes, presence of 24-isopropylcholestane and an abundant series of mid-chain methylalkanes – sometimes termed
X-peak compounds (Grantham et al., 1987; H€
old et al.,
1999; Love et al., 2008; Grosjean et al., 2009). Oils, in
particular from the Eastern Flank Play, have wide ranges of
densities (generally high) and viscosities, and crudes exhibit
very low gas/oil ratios (Al-Marjeby & Nash, 1986). Oils
with the same properties and of similar age are also known
from successions in Pakistan, Eastern Siberia, India and the
Eastern European Platform (H€
old et al., 1999). In TM-6,
bitumen and kerogen extracted from whole-rock powder
have similar d13C values and show comparable d13C trends
with stratigraphic height (Fig. 3A). Kerogen, in general, is
slightly 13C-depleted relative to coexisting bitumen
(Fig. 3A) in Nafun Group samples – a feature observed in
Neoproterozoic-age strata globally, but largely reversed by
late Ediacaran time (Logan et al., 1995; H€
old et al., 1999;
Kelly, 2009). The largest fractionation between bitumen
and kerogen is 6.3& and occurs in the middle of the Shuram Formation, whereas latest Ediacaran and early Cambrian samples from the Ara Group show similar values for
bitumen and kerogen (within the reproducibility). In
general, the carbon isotopic composition of bitumen and
© 2013 John Wiley & Sons Ltd
Organic carbon through the Shuram Formation
δ13Corg‰VPDB
X-peak/nCx
–40 –37 –34 –31 –28
ARA
A5-A6
A
0
B
0.5
1
Methylhopane index (%)
1.5
2
C
0
D
5
10
15
%24-isopropylcholestane
0
20
E
7
F
1
2
3
4
G
A4
A0-A3
4b
Shuram
100 m
NAFUN
SE + recovery
4a
Buah
SE
4c
4d
Khufai
Masirah
Bay
bitumen
kerogen
C22
C24
2αMe
3βMe
Hadash
–0.2 –0.1 0.0
Limestone
Siltstone & shale
Dolostone
Sandstone
0.1
0.2
PC1
0.4 0.6 0.8
1
1.2
Sterane/Hopane
60
65
70
75
80
%C29 steranes
Fig. 3 Carbon isotopic and lipid biomarker stratigraphic profiles from the TM-6 drill well. (A) d13Corg values for bitumen (filled diamonds) and kerogen (open
squares). (B) First principle component (PC1) from principal component analysis of C19-C26 n-alkane abundances. PC1 explains 91% of the variance in the
data set; variable loadings are C19 (0.6328), C20 (0.3610), C21 (0.2050), C22 (0.0446), C23 ( 0.1698), C24 ( 0.2820), C25 ( 0.3537), C26 ( 0.4379). (C)
Relative X-peak mid-chain methylalkane abundances measured by X-peak/n-alkane for C22 (yellow) and C24 (blue) homologues. (D) Sterane/hopane (using 2
isomers of diasteranes and 4 isomers of C27-C29 regular steranes/19 isomers of C27-C35 hopanes; Cao et al., 2009). (E) Methylhopane indices for 2a-methylhopane (filled diamonds) [(2-methyl C30 hopane/2-methyl C30 hopane + C30 ab hopane) 9 100)] and 3b-methylhopane (open squares) [C31 3b-methylhopane/(C30 ab methylhopane + C31 3b-methylhopane) 9 100]. (F) Relative abundance of C29 steranes to total C27-C29 steranes, in per cent (4 isomers of C29
regular steranes/4 isomers of C27-C29 regular steranes 9 100). (G) Relative abundance of 24-isopropylcholestane to total C27-C29 steranes, in per cent (24isopropylcholestane/2 isomers of diasterane and 4 isomers of C27-C29 regular steranes 9 100). Pale blue shaded panel indicates the Shuram excursion (SE)
and recovery; the line in panel A represents the +5& to 12& excursion.
kerogen shows a very similar trend to the bulk organic
carbon values (Fig. 2D); neither tracks the negative carbon
isotope excursion observed in carbonates (Fig. 2B).
Lipid biomarker data from TM-6 bitumens exhibit characteristics similar to Nafun strata and oils studied in TM-6
and elsewhere in Oman (e.g. Grosjean et al., 2009; Love
et al., 2009), but also show intraformational differences
and trends with stratigraphic height. The abundances of
C19-C26 n-alkanes provide insight into bacterial vs. eukaryotic contributions to sedimentary organic matter. These
larger alkanes are not strongly affected by evaporative
losses during sample handling, and although they can be
modified by biodegradation, this process did not substantially impact the biomarker composition of pre-salt oils
(Grosjean et al., 2009). The relative abundances of C19C26 n-alkanes, statistically reduced to the first principle
component (which explains 91% of the data variance) using
principle component analysis, highlight fundamental differences in the organics preserved within the stratigraphy
(Fig. 3B). The low-molecular-weight end of the spectrum
receives strong positive loadings on PC1, whereas the
© 2013 John Wiley & Sons Ltd
higher-molecular-weight alkanes receive negative loadings.
Consequently, decreasing PC1 values imply greater contributions of algal biomass to sedimentary organic matter –
an inference weakly supported by sterane to hopane ratio
data (discussed below).
X-peaks are an unusual series of C14-C30 mid-chain
monomethyl branched alkanes, but with greatest abundance in the C20-C26 range, which have been detected
before in Huqf oils and sedimentary rocks (Klomp, 1986;
H€
old et al., 1999; Grosjean et al., 2009) as well as in
other late Proterozoic–early Cambrian samples. These
include heavy petroleum from Pakistan (Grantham et al.,
1987), source rocks and oils from the Eastern European
(Russian) Platform (Bazhenova & Arefiev, 1996) and oils
from the Eastern Siberian Platform (Fowler & Douglas,
1987). Ratios of mid-chain monomethyl-branched alkanes
to n-alkanes (n-Cx) show very high values throughout the
Nafun and Ara groups and several systematic excursions
with local maxima recorded in the lower and middle
Shuram Formation and local minima in the middle Buah
Formation, respectively (Figs 3C and 4). In general,
8
C. LEE et al.
m/z 85
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Time (min)
mid-chain monomethyl branched alkane compounds are
much less abundant (ratios <0.1 relative to n-alkane with
the same carbon number) in Phanerozoic rocks (Peters
et al., 2005). However, the observation of the C20-C26
pattern (X-peaks) with a slight even-over-odd carbon number preference is only prominent in late Neoproterozoicto early Cambrian-age sedimentary rocks and oils (Love
et al., 2008). For typical Phanerozoic strata, the methylalkane abundance tracks n-alkane abundance, where small
amounts of methylalkanes are partially derived from diagenesis of linear alkanes. Patterns in late Neoproterozoic
and early Cambrian samples with prominent X-peaks do
not simply record n-alkane abundance patterns (Figs 3B,C
and 4), implying a specific and significant biological source
input for these compounds. The biological sources of
X-peak compounds remain unconfirmed at present.
Although rare, elevated concentrations of mid-chain
monomethyl branched alkanes are observed in unusually
organic sulphur-rich Jurassic rocks (e.g. van Kaam-Peters
& Sinninghe Damste, 1997) and suggest these compounds
might be derived from organisms living in benthic microbial mats associated with stratified bottom waters, perhaps
sulphur-oxidising chemoautotrophic bacteria (Love et al.,
2008). If correct, this implies a substantial contribution to
sedimentary organic matter from organisms that would
have fractionated carbon isotopes quite differently than
expectations from ribulose-1,5-bisphosphate carboxylase
oxygenase in the Calvin–Benson–Bassham cycle of embryophytes (e.g. Scott et al., 2004). Compound-specific measurements of X-peak compounds reveal a 13C-depletion by
an average of 3.7& (up to 6.5& for C24) when compared
54
28
29
56
58
Fig. 4 Selected
partial
m/z
85
ion
chromatograms of n-alkanes (filled circles) and
mid-chain monomethyl alkane series (X-peak
compounds; open circles) denoted by their
respective carbon numbers through the
Shuram excursion. X-peak/nCx ratios were
calculated by integration in m/z 85
chromatograms for compounds of the same
carbon number (e.g. X-24/nC24). Each
chromatogram is labelled with a letter
corresponding to their relative stratigraphic
position displayed in Fig. 3C. The horizontal
axis represents elution time (in minutes), and
the vertical axis is relative intensity.
to n-alkanes in Huqf bitumens (H€
old et al., 1999), implying that the source organisms either fractionated carbon
isotopes to a greater degree or were benthic and consumed
13
C-depleted bottom water DIC (Love et al., 2008).
X-peak/n-Cx ratios correlate with the d13C values of bulk
organic carbon, kerogen and to a lesser degree, bitumen.
It is interesting, however, that they do not correlate well
with D13carb-org. Uncertainty about sources aside, the high
X-peak abundances observed in TM-6 strata (up to 60%
more abundant than the similar n-alkane) are exotic and
imply a specific and significant source input. These data
also highlight the ever more apparent prevalence of these
compounds in the late Neoproterozoic stratigraphic record
globally.
Sterane (C27-C29) to hopane (C27-C35) ratios provide a
coarse proxy for the relative contributions of bacterial and
eukaryotic biomass to preserved sedimentary organic
matter. Generally, sterane/hopane ratios increase upsection, but also show significant variability with several notable outliers at the top of the Masirah Bay Formation and
within the lower Shuram Formation (Fig. 3D). The ranges
in sterane/hopane ratios observed here (between ca. 0.4
and 1) imply significant changes in the autotrophic communities through the Shuram excursion, which coincide
with the facies and environmental changes observed upsection. These values still remain within the second quartile
of the typical range (0.5–2.0) observed in Phanerozoic
marine sedimentary rocks and oils (Peters et al., 2005).
Although bacteria and eukaryotes can fractionate carbon
isotopes differently (due to differences in cell size and
physiology; as discussed in Close et al., 2011), we do not
© 2013 John Wiley & Sons Ltd
Organic carbon through the Shuram Formation
observe relationships between sterane/hopane and the isotopic composition of total organic carbon, bitumen, kerogen or D13carb-org.
Methylhopane indices show stratigraphic variability
through the Shuram excursion. 2a-methylhopane indices,
describing the relative abundance of compounds largely
derived from cyanobacteria and a-proteobacteria (Summons
et al., 1999; Welander et al., 2010), vary with depth, but
are high throughout much of TM-6 and can exceed 15%
(Fig. 3E). These values are consistent with previous
observations of microfossils and lipid biomarkers from
Proterozoic strata that suggest overall proportionally
higher amounts of primary production from bacteria during Neoproterozoic time than is observed from the late
Paleozoic through today (Summons et al., 1999; Knoll
et al., 2007). The relative abundance of 3b-methylhopanes,
compounds largely produced by microaerophilic type 1
methanotrophic proteobacteria and acetic acid bacteria
(Farrimond et al., 2004), also vary with depth exhibiting
similar overall pattern to 2a-methylhopanes. Despite the
stratigraphic variability, the lack of a major secular change
in the relative abundance of methylhopanes through the
Shuram excursion suggests the absence of extreme temporal shifts in local environmental conditions, for example
temperature, salinity, and sedimentary methane fluxes
(Rohrssen et al., 2013).
Sterane patterns in TM-6 bitumens are dominated by
C29 isomers (relative abundances of >70%; Fig. 3F),
hydrocarbons derived from sterols typical of green algae
(Volkman, 1986). This feature is also characteristic of
Proterozoic and Paleozoic bitumens and supports the
notion that green algae were important primary producers
prior to the Mesozoic marine revolution of higher order
endosymbiotic clades (e.g. Grantham & Wakefield, 1988;
Schwark & Empt, 2006; Knoll et al., 2007). These observations may also provide some additional context into the
possible taxonomic identities of the leiosphaerid acritarchs
observed throughout Nafun Group strata (Butterfield &
Grotzinger, 2012).
Finally, we observe variable but significant abundances
of 24-isopropylcholestanes (from 1 to 3% of summed C27C29 abundances, typical for South Oman oils and rocks)
throughout the Nafun Group and Ara Group stratigraphy
(Fig. 3G). These biomarkers are thought to reflect the
diagenetically stabilised equivalents of C30 sterols produced
by marine demosponges, and extend into the Cryogenianage Abu Mahara Group elsewhere in Oman, constituting
the oldest observations of Metazoa (Love et al., 2009).
From comparative biology, it was recognised that early
sponges had an aerobic metabolism (e.g. Berkner &
Marshall, 1965; Towe, 1970; King et al., 2008) – anaerobic
metazoans appear evolutionarily derived (Danovaro et al.,
2010). The facies and source-rock characteristics of this
deep-water paleoenvironment suggest that the seafloor in
© 2013 John Wiley & Sons Ltd
9
this sub-basin was anoxic through much of Nafun Group
time. If 24-isopropylcholestanes were produced by adult
benthic organisms with similar growth habits and ecology
to modern demosponges (e.g. Sperling et al., 2011), then
these molecules were likely advected to deeper sites from
organisms living on the shelf. Alternatively, these observations could be explained by the broadcast spawning of
planktonic larval stages or an early-evolved sponge group
with a largely planktonic life cycle. If the latter is correct,
the sedimentary context observed here provides some
support for the hypothesis that the evolution and development of sponge larvae may have been an important
waypoint in the evolution of higher metazoan taxa
(Nielsen, 2008).
IMPLICATIONS FOR CARBON CYCLE
FUNCTION THROUGH THE SHURAM
EXCURSION
The Shuram excursion presents a suite of challenges to
our understanding of the limits of the operation of the
Neoproterozoic carbon cycle. This has engendered
hypotheses that range from the existence of a large DOC
reservoir (Rothman et al., 2003; Fike et al., 2006), to the
alteration via known (Knauth & Kennedy, 2009; Derry,
2010a; Schrag et al., 2013) and unknown (Grotzinger
et al., 2011) diagenetic processes. Though diagenetic
hypotheses for the Shuram excursion provide a simple
explanation for decoupled carbonate and organic carbon
isotope records, we do not observe the covariation of 13C
and 18O isotope ratios in TM-6 carbonates through the
Shuram excursion (Fig. 5A) anticipated by these diagenetic
hypotheses (Knauth & Kennedy, 2009; Derry, 2010a).
Although both the absolute values and variation seen in
d18O throughout the section point to significant water–
rock interaction, this alteration does not appear to have
significantly affected the d13C isotope composition. It is
also important to note that the overall magnitude and
shape of the Shuram excursion are the same across Oman
– from TM-6 in the south to the Huqf area to the Oman
Mountains (Burns & Matter, 1993; Le Guerroue et al.,
2006c) – despite the dramatic differences in depositional
environment (e.g. water depth), sedimentary organic
carbon input and burial history between these regions.
The differences in burial history of Huqf strata across
Oman are substantial; this is suggested by the varying
d18Ocarb values across Oman, but not reflected in the
d13Ccarb values (Burns & Matter, 1993; Le Guerroue
et al., 2006c). This pattern is inconsistent with burial diagenesis hypotheses for the d13Ccarb excursion (e.g. Derry,
2010a). Further, it was recently hypothesised that the
Shuram excursion (and the Neoproterozoic carbon isotope
record more broadly) reflects the local influence of DIC
and alkalinity derived from anaerobic respiration metabo-
10
C. LEE et al.
TOC (wt%)
Carbonate (wt%)
0
20
40
60
80
0
100
3
6
9
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15
δ13Ccarb‰VPDB
10
5
0
SE
–5
–10
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–15
–15
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–10
–5
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5
δ18Ocarb ‰VPDB
10
C
–45
D
–40
–35
–30
–25
δ13Corg ‰VPDB
–20
E
20
25
30
35
40
45
Δ 13Ccarb-org ‰
Fig. 5 TM-6 carbonate carbon isotope ratio (d13Ccarb) cross-plots plotted against (A) Carbonate oxygen isotope ratios (d18Ocarb &VPDB); R2 = 0.2 for data
points through the Shuram excursion as indicated. (B) Carbonate abundance in wt%; R2 = 0.09. (C) Organic carbon isotope ratios (d13Corg); R2 = 0.2. (D)
Total organic carbon in wt%; R2 = 0.08. (E) D13Ccarb-org &. The only significant relationship in the cross-plots shown occurs in panel E (R2 = 0.55).
lisms driven by the biological pump and sedimentary
organic matter inputs (Schrag et al., 2013). We note that
despite the high sedimentary organic carbon loadings present in this part of the basin (which is markedly different
than other Oman sections; e.g. Fike et al., 2006), the
d13Ccarb excursion is similar (Fig. 5D) – observations that
do not support the hypothesis of Schrag et al. (2013) for
the Shuram excursion. For these reasons, we tentatively
regard the carbonate carbon isotope excursion as representative of the time-varying behaviour of marine DIC.
Examining the time series of Dcarb-org, it is clear that
during the onset of the Shuram excursion, carbonate and
organic phases were decoupled with regard to their isotopic composition (Fig. 2F). However, from the middle
Shuram Formation through the Buah Formation and into
the Ara Group – through the latter part of the Shuram
excursion recovery – d13Ccarb and d13Corg show coupled
behaviour (Fig. 2F). This is similar to shallower sections
in Oman (Fike et al., 2006) and was also observed in
Doushantuo unit IV sections from South China (McFadden
et al., 2008). These observations (including organic carbon isotope ratios as low as 39&) support the idea
that the Shuram excursion records a primary carbon cycle
signal, as the stratigraphic coherence of this covariation is
unlikely to occur simply by chance. In general, the
disparity between the paired inorganic and organic data
sets has been a focal point for hypotheses regarding the
nature of Neoproterozoic sedimentary organic matter,
suggesting that organic matter could be low in concentration (Jiang et al., 2012; Johnston et al., 2012), buffered by a large, metastable, DOC reservoir (Rothman
et al., 2003), or have mixed (Melim et al., 2004; Swart,
2008), multiple (Oehlert et al., 2012) or exogenous
(Kaufman et al., 2007; Johnston et al., 2012) carbon
sources.
Lipid biomarker and bitumen data from TM-6 support
hypotheses that the bulk isotopic composition of sedimentary organic matter reflects multiple syngenetic sources.
Organic matter composition and source inputs show stratigraphic variation through the Shuram excursion. In addition, the carbon isotopic composition of kerogen and
bitumen display remarkably similar stratigraphic patterns.
This discounts hypotheses wherein the organic matter was
derived from either a large isotopically homogenous DOC
reservoir (e.g. Rothman et al., 2003) or exogenous,
migrated or detrital fossil carbon sources. The isotopic
compositions of organic phases (bitumen and kerogen)
from TM-6 vary stratigraphically, but not as a function of
organic carbon concentrations. Furthermore, the difference
between d13Ccarb and d13Corg (here Dcarb-org, Fig. 2F) does
not vary as a function of organic carbon content (TOC wt
%) – two variables that Johnston et al. (2012) hypothesised
should show a hyperbolic relationship due to compositional mixing of syngenetic and fossil detrital organic carbon sources in sections with decoupled carbonate–organic
records. Thus, we can effectively rule out the hypotheses
that specify inputs from weathering of fossil detrital carbon
sources (Kaufman et al., 2007; Johnston et al., 2012) to
explain this excursion.
Strong evidence for non-migrated, syngenetic organic
matter is demonstrated by two critical observations. First,
we report a combination of variable magnitude and distinctive biomarker characteristics (e.g. abundance of distinct Xpeak mid-chain methylalkane series and high C29 (green
algal) and C30 (marine demosponge) steranes) in TM-6
samples, as found in all Huqf Supergroup oils and rocks
from the South Oman Salt Basin. Similar compositional
characteristics were observed from late Neoproterozoic-age
eastern Siberian oils (Kelly et al., 2011). Second, the
strong relationship between bitumen and kerogen is
observed compositionally through comparisons between
bitumen and kerogen hydropyrolysates (Love et al., 2008,
2009; Grosjean et al., 2009). This is also reflected in the
similarity of their d13C values (m = 1.1; R2 = 0.61), which
is congruent with non-migrated marine-derived organic
matter.
© 2013 John Wiley & Sons Ltd
Organic carbon through the Shuram Formation
Models of the Neoproterozoic carbon cycle that predict
the isotopic composition of sedimentary organic matter as
a linear translation of carbonate carbon isotope time-series
data probably are too simple to accurately describe the
carbon cycling at this time, particularly during large perturbations. Organic carbon isotope ratios need not provide a
global proxy and can show idiosyncratic local differences
depending on the diversity of biological communities and
their respective carbon utilisation pathways (Pancost &
Sinninghe Damste, 2003). Consequently, organic carbon
isotopic trends can appear strongly decoupled from trends
in carbonate carbon for primary biogeochemical and ecological reasons (e.g. Pancost et al., 1998). The clear shifts
in organic matter source contributions is evidenced by the
notable variations in a wide range of lipid biomarker abundances through the Nafun Group of TM-6, particularly
during the Shuram excursion. This underscores the biological richness underlying stratigraphic organic carbon isotope ratio data through the largest carbon isotope
excursion in the geologic record.
If primary, the Shuram excursion provides an important
isotope mass balance constraint on the middle to late Ediacaran carbon cycle. But in considering mechanisms for
the excursion, it is useful to evaluate the quality of the
assumptions commonly made in these interpretations.
Most isotope mass balance frameworks for the geological
carbon cycle make three assumptions (often implicitly) to
solve for the burial fraction of organic carbon: steady state
(where inputs and outputs are equal), the isotopic composition of the inputs and the isotopic difference between
coeval carbonates and organic matter (Hayes et al., 1999).
Although dynamics are required by the excursion itself,
the long timescales suggested by the substantial stratigraphic thickness of the Shuram excursion support quasistatic interpretations of the data (e.g. Rothman et al.,
2003). The isotopic composition of historical carbon cycle
inputs cannot be directly observed and is often set
between 5 and 7&. But it is important to note that
mantle carbon is highly variable and bimodal (one mode
at 25& and another at 6&) in its isotopic composition
(Deines, 2002), nearly displaying the range observed in
sedimentary rocks. In addition to the variable isotopic
composition of outgassing, the likelihood of changes in
the isotopic composition of the inputs is assured by the
weathering of pre-existing sedimentary rocks (e.g. Halevy
et al., 2012). The Shuram excursion requires input values
lower than
12&. But values this low for historical
carbon inputs are, in principle, reasonable. Isotopic differences between carbonates and organic carbon are often set
between 25 and 30& and assumed to be constant. In
principle, however, this difference (as D13Ccarb-org) is
observable in sedimentary rocks. As stated above, organic
carbon isotope ratios need not record global trends. Nevertheless, several Shuram-age sections with paired records
© 2013 John Wiley & Sons Ltd
11
capture a secular increase in the observed D13Ccarb-org (e.g.
Calver, 2000; Fike et al., 2006; McFadden et al., 2008),
suggesting a need to interrogate a different hypothesis for
the origin of this excursion – one wherein the d13Ccarb values were largely controlled and driven by global changes
in carbon isotope fractionations between coeval carbonate
and organic carbon (e.g. Rothman et al., 2003). Mass balance can be satisfied with multiple combinations of lower
input values, reduced organic carbon burial fluxes and
lower fractionations. But if the observed changes in
D13Ccarb-org are indeed global in scope, mechanisms
behind why these fractionations might change so systematically remain unclear.
CONCLUSIONS
Paired data sets of d13Ccarb and d13Corg are largely
decoupled during the Shuram excursion, even in highly
organic-rich strata that characterise deeper-water paleoenvironments of the Nafun Group in Oman. Although the
well-defined d13Ccarb decline at the beginning of the
Shuram excursion is not recorded in the carbon isotopic
composition of any of the organic phases (TOC, bitumen,
and kerogen), these materials show systematic trends that
define a broad negative excursion, the onset of which
occurs within the Khufai Formation and ends with a recovery that matches d13Ccarb in the middle to upper Shuram
Formation. Neither the carbon isotopic composition of the
organic phases nor the difference between d13Ccarb and
d13Corg varies as a function of organic concentration. Lipid
biomarker data reveal substantial changes in the biological
communities and organic matter source inputs through the
Shuram excursion in this sedimentary basin, but within the
ranges expected of Neoproterozoic sedimentary rocks.
Together these observations imply that carbonate–organic
isotopic decoupling during the Shuram excursion is not a
result of mixing of fossil or exogenous carbon sources
(either DOC, detrital or migrated) with syngenetic organic
matter, although differential mixing of distinct syngenetic
sources may have played an important role in the observed
differences between inorganic and organic carbon isotope
ratios. Ultimately, these results highlight the possibility
that systematic global changes in the fractionations
between organic and inorganic carbon provided a driving
mechanism for the Shuram excursion.
ACKNOWLEDGMENTS
We acknowledge the Agouron Institute for supporting this
work, Petroleum Development Oman for access to subsurface materials and Kristin Bergmann for helpful comments.
Additionally, we acknowledge editors Roger Buick and
Kurt Konhauser for handling the manuscript and constructive comments provided by three anonymous reviewers.
12
C. LEE et al.
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